Non-volatile memory devices are an important element of integrated circuits due to their ability to maintain data absent a power supply. Phase change materials have been investigated for use in non-volatile memory cells. Phase change memory (“PCM”) cells include phase change materials, such as chalcogenide compounds, which are capable of stably transitioning between physical states (e.g., amorphous, semi-amorphous, and crystalline states). Each physical state exhibits a particular resistance that may be used to distinguish logic values of the memory cell.
To change the physical state of the phase change material of a PCM cell, an electrical current is applied to a heating element coupled to the phase change material. The amount of current required to sufficiently alter the physical state of the phase change material is related to the size of an area comprising an interface between the heating element and the associated phase change material. Thus, more current is required to change the physical state of a phase change material in a device if the interface area is larger compared to a device with a smaller interface area between the heating element and the phase change material.
Some non-volatile memory devices include an array of PCM cells and selection elements for selecting certain PCM cells for reading and writing operations. The array may include PCM cells aligned in a first direction that share a common conductive data/sense line (e.g., a common conductive bit line), and aligned in a second direction that share a common access line (e.g., a common conductive word line). The PCM cells may be coupled to the bit lines using a conductive adhesion material. The selection element may be a vertical PNP bipolar junction transistor (BJT) including a common p-doped semiconductor collector, an n-doped semiconductor base, and a p-doped semiconductor emitter. BJT selectors enable controlled selection of desired PCM cells in the array by, in part, allowing for electrical current to flow in only one direction across an interface between the oppositely doped semiconductor regions, depending on the voltage of the applied current. The PCM cells may be coupled to respective p-doped semiconductor emitters.
The PCM cell arrays may include groups of PCM cells and associated bit lines separated by base contacts for electrically contacting and accessing the n-doped semiconductor base region. The bit lines and the base contacts are formed of a conductive material. To facilitate the formation of bit lines and base contacts that do not directly electrically communicate, the PCM cells immediately adjacent to the base contacts may be formed without a full conductive bit line. The PCM cells immediately adjacent to the base contacts and lacking a bit line may be referred to as so-called “dummy cells.” The conductive adhesion material coupled to the dummy cells may be referred to as so-called “dummy bit lines.” During normal operation of the PCM cell arrays, the dummy cells and the dummy bit lines are not electrically accessed and are not used to store information.